The preservation of biological substances to prevent irreversible cell damage involves a difficult problem, especially where the conservation of blood is concerned.
For some time, the preservation of blood has been practiced, by conventional techniques, inter alia by deep freezing and storage in liquid nitrogen. This technique permits the long-term storage of blood. A necessary precaution for the successful deep freezing of blood is, however, the admixing therewith of protective additives. Otherwise, when the temperature of the blood passes through the freezing range, the red blood cells are damaged and the cells are hemolized. So that, upon thawing of the preserved blood, it is not necessary to wash out the additives prior to use intransfusion, additives which are inocuous to the human organism, e.g. HES (hydroxyethyl starch), are used.
One of the problems which arise in the preparation of the blood for the freezing process is that of mixing as quickly and completely as possible the additive with the blood, since the cell-survival rate increases with increasing speed of introduction of the additive into the blood. This rate depends upon the penetration of the additive into and its deposition upon the cell membranes.
Furthermore, it is also necessary to ensure that all cells of the suspension have a closely as possible the same concentration of protective additive. Previous processes which have mixed blood and protective additives of the type described above by shaking have resulted in inhomogeneities which give rise to a high degree of damage to the red blood corpuscles during the freezing process.